MAMMALIA. 513 
Turning to the dentition there are sufficiently obvious 
differences. In the ox there are no incisors nor canines 
in the upper jaw, their place being taken by a horny pad. 
In the mandible there are three pairs of chisel-shaped 
incisors and a pair of canines which resemble incisors in 
shape and size. In the horse, on the other hand, there are 
three pairs of incisors in both upper and lower jaw which 
are of a peculiar shape. They have their terminal surface 
pushed in as a deep pit, partially filled with cement. On 
being worn flat the surface of the tooth presents two con- 
centric circles of enamel, the inner circle becoming narrower 
with age. The canines 
are small and pointed Fig, 352, Upper Jaw (LEFT-HALF) 
and are only rarely pre-op Younc (A) AND OLD Horsk (B). 
sent in the female. 
As regards the molar 
series we have seen that 
there are considerable 
resemblances in the two 
types, and in each there 
are six functional teeth 
on each side, of which 
three are premolars and sth 
three are molars. Here i) ly 
the resemblances end. te 
In most horses there is, eS) 
at least in theadolescent vi 
stage, a very small first 
premolar in each upper 
jaw, which usually falls out at maturity. Thus the full 
dentition of a young horse may be given as #4, but that of 
a mature mare is #33. The dentition of the ox is <3, 
Though the patterns of the enamel in the molars have 
a general resemblance, a little study shows that they are 
derived from different types. The horse starts from the 
simple bilophodont type, found in the tapir, consisting of 
a pair of transverse ridges: this is further complicated, as 
in the rhinoceros, by a junction of the two ridges and by 
their bending into a crescentic outline: in the horse these 
ridges are still further twisted, the multiplication of enamel 
‘tidges being the end in view. 
M. 34 
